This invention relates to a subscriber unit and method of cell selection for a cellular communication system and particularly a CDMA cellular communication system.
In a cellular communication system each of the subscriber units (e.g. mobile stations) communicates with typically a fixed base station. Communication from the subscriber unit to the base station is known as uplink and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.
As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks, the base stations themselves and various controllers required for operating the network. A call from a subscriber unit is routed through the fixed network to the destination specific for this call. If the call is between two subscriber units of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other subscriber unit currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a subscriber unit and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the subscriber units and the base stations. This spectrum must be shared between all subscriber units simultaneously using the system.
One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA) communication system, the signals are prior to being transmitted multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be de-spread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of subscriber units can be accommodated in the same wideband spectrum by allocating different codes for different subscriber units. Codes are chosen to minimise the interference caused between subscriber units typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in xe2x80x98Spread Spectrum CDMA Systems for Wireless Communicationsxe2x80x99, Glisic and Vucetic, Artech house Publishers, 1997, ISBN 0-89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.
In order to efficiently use the available resources it is essential that the cell selection is optimised. If subscriber units are served by cells which are not the best serving cell, the transmit powers will be increased in order to compensate and the interference will increase thereby reducing the capacity of the system. Cell selection is therefore very important both when setting up a new call or when selecting appropriate cells for handovers. UMTS and other 3rd generation wideband-CDMA (W-CDMA) communication systems rely on a random access channel (RACH) for setting up calls. In W-CDMA, the physical RACH (PRACH) channel is used not only to set up calls, i.e. to send requests for a dedicated traffic channel, but also to transmit short data packets.
The ETSI document UMTS XX.07 UTRA Layer 1 Description. FDD Physical Layer Procedures, Version 0.2.0 gives a description of the cell selection and access procedure currently envisaged in UMTS. In summary, in the known wideband-CDMA random access method the mobile always communicates with a single base station, and this base station is selected on the basis of the maximum downlink pilot strength.
During the random access procedure, in its calculations of the necessary backoff after a failed access attempt, the mobile also makes use of so called dynamic persistence factor which is also published on the BCCH. This parameter, as described in the ETSI document xe2x80x9cMechanisms for Managing Uplink Interference and Bandwidthxe2x80x9d Tdoc SMG2 UMTS-L1 683/98, is a means of controlling the uplink data flow, and it represents an indicator of congestion on the RACH. The dynamic persistence factor indicates the duration that a subscriber unit should wait before transmitting a new RACH following a failed access attempt.
However, known cell selection methods have significant disadvantages as they optimise the downlink performance without considering the uplink performance. This results in inefficient cell selection, which reduces the communication capacity of the communication system.
The inventor of the current invention has realised that cell selection based on only downlink pilot signal levels results in inefficient and sub-optimal cell selection. He has further realised that these effects can be mitigated by including uplink information in the cell selection process.
Accordingly to the present invention there is provided a subscriber unit for use in a cellular communication system having a plurality of base stations supporting at least said subscriber unit and transmitting a pilot signal and uplink characteristics information, the subscriber unit comprising: receiving means for receiving the uplink characteristics information from a target set of base stations selected from set plurality of base stations; base station selection means for determining at least one serving base station in response to said uplink characteristics information; and transmitting means for accessing the cellular communication system through said at least one serving base station.
Preferably, the subscriber unit further comprises means for determining a received pilot signal quality indicator in the subscriber unit and the determination of the at least one serving base station is in response to both the uplink characteristics information and the pilot signal quality indicator.
According to different features of the invention the uplink characteristics information can for example be an uplink interference level, an uplink congestion indicator or a is a downlink transmit power level indicator.
According to one feature of the invention, the subscriber unit comprises means for calculating an access message transmit power level in response to said uplink characteristics information and the base station selection is in response to said calculated access message transmit power level.
According to a different aspect of the invention, there is provided a method of cell access in a cellular communication system having a plurality of base stations supporting at least said subscriber unit and transmitting a pilot signal and uplink characteristics information, the method comprising the steps of: receiving the uplink characteristics information from a target set of base stations selected from set plurality of base stations in the subscriber unit; determining at least one serving base station in response to said uplink characteristics information; and the subscriber unit accessing the cellular communication system through said at least one serving base station.